I also had 3 treatments with brachytherapy and think that it is worth doing. It is somewhat uncomfortable and very undignified for 10-15 min each time. I had no immediate side effects. After a month or so you need to either start using a vaginal dilator or engage in sexual activity to avoid long term issues.

If there are stray cancer cells in your body, diet and lifestyle probably won't do much to prevent recurrence.

If your cancer was Grade 1 or 2, a recurrence would probably still be curable, but that's still really worth avoiding if possible.